No. In ex vivo human skin, oleic acid and oleyl alcohol at 0.75% produced almost identical 24-hour delivery of diclofenac diethylamine — 2.6-fold and 2.8-fold over control — yet only oleic acid raised transepidermal water loss and left skin electrical impedance incompletely recovered 24 hours after removal. Equal delivery, unequal barrier cost.
Published 30 September 2026 · Last reviewed 30 September 2026
The terms, defined
A chemical penetration enhancer (also called a permeation enhancer or accelerant) is an excipient added to a topical formulation to increase how much of a permeant crosses the stratum corneum, by increasing its partitioning into the barrier domain or its diffusion within it. Recognised mechanisms include fluidising the stratum corneum lipid bilayers, interacting with intercellular proteins, disrupting or extracting intercellular lipids, raising the thermodynamic activity of the permeant, and increasing stratum corneum hydration (Alkilani, McCrudden and Donnelly, 2015).
The stratum corneum is the outermost skin layer and the rate-limiting barrier to topical delivery, principally because of its highly hydrophobic extracellular lipid matrix.
Transepidermal water loss (TEWL) is the rate at which water diffuses outward through the epidermis, measured in g·m⁻²·h⁻¹. It is a functional index of barrier integrity: a rise indicates a more permeable barrier.
Skin electrical impedance is the opposition the skin offers to an alternating current. Lower impedance means less opposition, and is read as a second, independent index of barrier perturbation.
A fold change (enhancement ratio) is the amount permeated with the enhancer divided by the amount permeated without it, under otherwise identical conditions.
What the experiment actually measured
Kováčik and colleagues (2023) compared two enhancers that share an 18-carbon chain and a central cis double bond, and differ only in their head group: oleic acid, a monounsaturated ω-9 fatty acid, and oleyl alcohol, the corresponding fatty alcohol. Both were used at 0.75% w/v — the concentration found in commercially available topical gels — in a vehicle of isopropyl alcohol, propylene glycol and water (20:6:74, v/v/v), carrying 2.32% diclofenac diethylamine (equivalent to 2% free diclofenac).
Permeation was measured in Franz diffusion cells on dermatomed ex vivo human skin of approximately 400 µm, 1 cm² permeation area, acceptor phase phosphate-buffered saline at pH 7.4, water bath at 32 ± 1.0 °C, without occlusion. Twelve skin fragments from three donors were used for the single-dose experiment. Barrier integrity was tracked separately by TEWL and electrical impedance, and the molecular interaction by Fourier-transform infrared spectroscopy using deuterated enhancers.
If you want the measurement method itself rather than its results, we have described it separately in how skin penetration is actually measured.
The two enhancers, side by side
| Measurement | Control (vehicle alone) | Oleyl alcohol 0.75% | Oleic acid 0.75% |
|---|---|---|---|
| Cumulative diclofenac in acceptor, single dose, 24 h | 8.2 ± 5.8 µg/cm² | 23 ± 3.1 µg/cm² (2.8-fold) | 21 ± 9.2 µg/cm² (2.6-fold) |
| Share of the applied dose (232 µg/cm²) | 3.5% | 9.9% | 9.1% |
| Cumulative, four doses every 12 h, 48 h | 50 ± 24 µg/cm² | 135 ± 46 µg/cm² (2.7-fold) | 153 ± 31 µg/cm² (3.1-fold) |
| Share of the applied dose (4 × 232 µg/cm²) | 5.4% | 14.5% | 16.5% |
| TEWL after 24 h at 150 µL/cm² | reference | rise smaller than oleic acid | highest rise of the three |
| TEWL after removal of the sample | returns to pre-application values | returns to pre-application values | returns to pre-application values |
| Electrical impedance | reference | not adversely affected | significantly affected; not completely reversed 24 h after removal |
The first row is the one worth pausing on. Over 24 hours the two enhancers are, for practical purposes, interchangeable as delivery agents: the difference between 23 and 21 µg/cm² sits well inside the dispersion of the measurement. The difference between them is not how much gets through. It is what the skin looks like afterwards.
Two mechanisms, in sequence
The infrared data resolve the enhancement into two phases. In the first hours, oleic acid fluidises the stratum corneum lipids and moves diclofenac faster than oleyl alcohol does; the permeation profiles of the two differ considerably between roughly 2 and 8 hours. After about 12 hours that difference levels off and the fluidisation is no longer evident.
What persists is a second mechanism. The CD₂ stretching vibrations of the deuterated enhancers appear at approximately 2097 cm⁻¹ — virtually identical to those of the neat compounds — which confirms that the enhancers have phase-separated within the lipid matrix and formed enhancer-rich fluid domains. Those disordered domains, and the interfaces between them and the surrounding native lipids, provide the easier route. The authors conclude that after 24 hours of application this domain formation, rather than generalised fluidisation, is the principal mechanism for both compounds.
That is also where the two part company. Oleic acid accumulated to a higher relative concentration in the stratum corneum lipids than oleyl alcohol, and was still detectable 12 hours after removal from the skin surface. The authors offer this as the reasonable explanation for why the impedance effect of oleic acid, unlike that of oleyl alcohol, does not fully reverse.
The trade-off, stated plainly
The finding above is a favourable case, not the general rule. The general rule, as the transdermal literature states it, is less comfortable: the major limitation for penetration enhancers is that their efficacy is often closely correlated with the occurrence of skin irritation (Alkilani, McCrudden and Donnelly, 2015). The same review notes that enhancers have been shown, in a limited number of cases, to cause local inflammation, erythema, swelling and dermatitis, and that their ability to increase transport across the skin is often low and variable.
Oleic acid illustrates the dose dependence of that trade-off. Skin irritation has been reported for oleic acid at 4.5–10%; at 0.75%, the concentration used in the study above and in marketed gels, the adverse signal is confined to a measurable, largely reversible perturbation of two barrier indices. Oleyl alcohol at up to 10% induced no discernible change in the histological appearance of nude mouse skin.
The practical inference for formulation is narrow and worth stating exactly: where two enhancers deliver comparably, the one that leaves the barrier closer to where it started is the better choice — and that question cannot be answered from the delivery figure alone. It requires a second measurement, taken after the product has been removed.
What this evidence does not license
Three limits apply to the study above, and they are worth naming because they bound what may legitimately be inferred from it.
- The permeant was a drug, not a cosmetic active. Diclofenac diethylamine has a molecular weight and lipophilicity suited to transdermal transport. Enhancement ratios do not transfer between permeants: the same enhancer can behave differently with a molecule of different size, charge or partition behaviour. The size and lipophilicity constraints are treated separately in our note on what determines whether a molecule penetrates skin.
- The barrier measurements used an excessive dose. The authors are explicit: at the 10 µL/cm² dose used for the permeation work, TEWL and impedance were not sensitive enough to detect any change. To differentiate the two enhancers at all they applied 150 µL/cm² — a dose they describe as excessive, chosen deliberately to expose the potential risk.
- The study was ex vivo, with three donors. The authors report it as a proof-of-concept study and present the data as mean ± standard deviation for that reason. Excised skin has no circulation, no inflammatory response and no repair kinetics.
None of this makes the result weak. It makes it specific — which is the condition under which a number is worth citing at all. The same discipline applies to the lipid matrix these enhancers act on: our open-access technical report on the pH constraint on L-ascorbic acid and the oxidative stability of tetrahexyldecyl ascorbate (CC BY 4.0) sets out how partitioning into that matrix governs what a lipophilic ester can and cannot do.
Scope
This article describes findings published in the peer-reviewed literature on chemical penetration enhancers in ex vivo human skin. It is not a claim about any product, and nothing in it should be read as an assertion that a cosmetic product prevents, treats or modifies any physiological condition. Cosmetic claims in the European Union are governed by Regulation (EC) No 1223/2009 and the common criteria of Regulation (EU) No 655/2013; in the United States, the cosmetic–drug boundary under the Federal Food, Drug, and Cosmetic Act is determined by the claim made, not by the ingredient used.
References
- Kováčik A, Kopečná M, Hrdinová I, Opálka L, Boncheva Bettex M, Vávrová K. Time-Dependent Differences in the Effects of Oleic Acid and Oleyl Alcohol on the Human Skin Barrier. Molecular Pharmaceutics. 2023;20(12):6237–6245. DOI 10.1021/acs.molpharmaceut.3c00648. PMID 37950377.
- Alkilani AZ, McCrudden MTC, Donnelly RF. Transdermal Drug Delivery: Innovative Pharmaceutical Developments Based on Disruption of the Barrier Properties of the Stratum Corneum. Pharmaceutics. 2015;7(4):438–470. DOI 10.3390/pharmaceutics7040438. PMID 26506371.
- TTS Clinical Scientific Team. Topical Vitamin C: the pH Constraint on L-Ascorbic Acid and the Oxidative Stability of Tetrahexyldecyl Ascorbate, v1.0. 2026. DOI 10.5281/zenodo.22491021.